Generally, exhaust gas coming out of an engine through an exhaust manifold flows into a catalytic converter mounted at an exhaust pipe and is purified therein. Then, noise of the exhaust gas is decreased while passing through a muffler, and the exhaust gas is emitted into the air through a tail pipe. The catalytic converter purifies pollutants contained in the exhaust gas. In addition, a particulate filter for trapping particulate matter (PM) contained in the exhaust gas is mounted in the exhaust pipe.
A denitrification catalyst (DeNOx catalyst) converter as the catalytic converter purifies nitrogen oxide (NOx) contained in the exhaust gas. If reducing agents, such as urea, ammonia, carbon monoxide, and hydrocarbon (HC), are supplied to the exhaust gas, the NOx contained in the exhaust gas is reduced in the DeNOx catalyst through oxidation-reduction reaction with the reducing agents.
Recently, a lean NOx trap (LNT) catalyst is used as such a DeNOx catalyst. The LNT catalyst absorbs the NOx contained in the exhaust gas when air/fuel ratio is lean, and releases the absorbed NOx and reduces the released nitrogen oxide and the nitrogen oxide contained in the exhaust gas when the air/fuel ratio is rich atmosphere (hereinafter, it will be called a ‘regeneration of the LNT’).
General diesel engines operate at the lean air/fuel ratio, however, it is required to artificially adjust air/fuel ratio to be the rich air/fuel ratio in order to release the absorbed NOx from the LNT. For this, a timing for releasing the NOx absorbed in the LNT (i.e., regeneration timing) needs to be determined.
In addition, if a temperature of the exhaust gas is high (e.g., the temperature of the exhaust gas is higher than 400° C.), the LNT cannot remove the NOx contained in the exhaust gas. In order to solve such problems, a selective catalytic reduction (SCR) catalyst together with the LNT is used.
If the LNT and the SCR catalyst are independently controlled, the following problems may occur due to different characteristics of the LNT and the SCR catalyst.
When the temperature at the upstream of the SCR catalyst is low, purification efficiency of the SCR catalyst may deteriorate and NOx exhaust may increase. In this case, the LNT instead of the SCR catalyst must remove the NOx.
In addition, the NOx is removed by the LNT when an NH3 amount stored in the SCR catalyst is high, the NH3 may be slipped from the SCR catalyst. In this case, the regeneration of the LNT must stop until the NH3 amount stored in the SCR catalyst is smaller than or equal to a predetermined amount.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention, and therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.